1. Field of the Invention
The present invention relates to a process for producing an “all-solid-state” Li-ion battery comprising composite ceramic electrodes by pulsed current sintering and to the “all-solid-state” battery obtained by such a process.
This invention is applicable to the manufacture of “all-solid-state” bulk electrochemical generators (as opposed to microbatteries).
2. Description of Related Art
Microbatteries are ultrathin “all-solid-state” batteries each element of which takes the form of a thin solid layer (layered ceramic materials). They generally consist of at least three layers, namely a negative electrode (anode), a positive electrode (cathode) and an electrolyte separating the negative electrode from the positive electrode and providing ion conductivity. Generally, lithium metal is chosen as the negative-electrode material. The materials used in the positive electrode are the same as in conventional lithium-ion batteries. The solid electrolyte is generally a vitreous oxide-based material, sometimes an oxysulfide or an oxynitride for a better ion conductivity.
Lithium-ion (Li-ion) batteries at the present time are used in most portable electronics on the market. Li-ion batteries have a number of advantages, especially:                they have no memory effect, in contrast to nickel-based accumulators;        they have a low self-discharge;        they do not require maintenance; and        they have a high energy density per unit mass. These batteries are therefore widely used in the field of mobile systems.        
“All-solid-state” Li-ion batteries, i.e. in which the two electrodes and the electrolyte are made of solid materials, are of great interest because of their potentially better properties relative to those of conventional batteries based on liquid or gel electrolytes. They especially provide a fundamental solution to the safety and environmental problems of conventional Li-ion batteries. Rechargeable batteries without a liquid electrolyte have considerable advantages including, for example, thermal stability, the absence of leakage and pollution, a high resistance to shocks and vibrations, a large window of electrochemical stability and an environmental impact when reprocessing the cells.
The various layers of microgenerators are mainly produced by physical vapor phase deposition methods such as cathode sputtering and thermal evaporation. The various layer are deposited in succession, thus making it possible to ensure the materials bond together and to create well defined interfaces. The development of bulk “all-solid-state” batteries frequently consists of a composite/electrolyte/Li-M metal alloy electrode multilayer in which the cohesion between the layers is most often ensured by simple cold pressing. Kitaura H. et al. (Journal of Power Sources, 2009, 189, 145-148) for example describe producing an “all-solid-state” Li—In/Li4Ti5O12 battery in which the electrolyte is produced by crystallizing the ceramic, the electrode and the electrolyte then being assembled by cold pressing. Sakuda A. et al. (Journal of Power Sources, 2009, 189, 527-530) moreover describe the production of a lithium secondary battery comprising an oxide-coated (Li2SiO3 and SiO2) LiCoO2 electrode and a ceramic electrolyte (Li2S—P2S5). In the process described in this article, the ceramic electrolyte layer is produced independently by heat treatment (210° C. for 4 hours). The positive electrode is produced using a mixture of LiCoO2 powder and ground ceramic electrolyte. The negative electrode is an indium foil. However, in this second case, forming the battery requires a number of steps and restrictive conditions since it is formed by cold compressing a positive electrode layer and the ceramic electrolyte and then applying the indium foil in an argon atmosphere in a glove box. In addition, this technique of forming the battery by cold pressing does not ensure high-quality interfaces between the layers, thereby imposing strong kinetic limitations meaning that thin electrodes must be used, these electrodes, because they are thin, containing little active material (less than 7 mg for an area of 0.79 cm2, i.e. less than 9 mg/cm2).
It has also been proposed before to produce thin (electrode and/or solid electrolyte) films by pulsed current sintering. Thus Xu X. et al. (Material Research Bulletin, 2008, 43, 2334-2341) describe producing a solid electrolyte with a NASICON-type structure (structure of the compound Na3Zr2Si2PO12) by pulsed current sintering of an Li1.4Al0.4Ti1.6(PO4)3 (LATP) nanopowder.
Nagata K. et al. (Journal of Power Sources, 2007, 174, 832-837) describe producing “all-solid-state” ceramic batteries by sintering. It is mentioned in this article that production of “all-solid-state” batteries comprising a solid crystalline-oxide electrolyte is difficult in that the heat treatment causes a reaction in the solid state between the electrolyte layer and the active electrode materials in contact with the electrolyte layer, thereby causing electrochemical deactivation of the electrolyte/electrode interface. The authors used a phosphate, such as Li1.3Al0.3Ti1.7(PO4)3 (LATP), for the solid electrolyte and a phosphate, such as LiCoPO4 et Li3Fe2(PO4)3, for the active electrode material; it is possible to carry out co-sintering without a chemical reaction taking place, the interface then remaining active. In this case, the sintering is carried out at 800° C. for 5 hours in air. The electrode material used according to the process described in this article does not however contain an electron-conductivity providing agent other than the electrode material, meaning that very small electrode thicknesses (smaller than 10 μm) must be worked with to obtain batteries having advantageous electrochemical properties, but the capacities of which are comparable to those of microbatteries.
Finally, document EP 2 086 038 describes an all-solid-state battery comprising a positive electrode, a negative electrode and, placed between the two is electrodes, a solid electrolyte. Each electrode comprises an active material (for example LiMn2O4 for the positive electrode or SiO2 for the negative electrode), an ion conductive agent (electrolyte) and an electron conductive agent, such as carbon or graphite. The electrolyte content does not exceed 30 wt % relative to the weight of the electrode. Specifically, this document teaches that if the electrolyte content and the content of electron conductive agent are too high, the amount of active material in each electrode will be reduced as a result and thus the capacity of the battery will be decreased. In addition, the electrode layers obtained are thin, about 12 to 15 μm in thickness. However, a small thickness also reduces the amount of energy that may be stored in a battery. This document also describes a process that allows such a battery to be obtained. This process has a number of steps consisting in manufacturing a positive electrode strip, a negative electrode strip and an intermediate electrolyte strip, separately. To do this, an acrylic (polymer)-based binder is used for each strip, which binder is then subsequently removed by burning. Next, the electrode strips are pressed against the electrolyte strip and the assembly is sintered.
There is therefore at the current time no method that allows an all-solid-state Li-ion battery with thick ceramic electrodes (for example about 30 μm or more in thickness) having very good electrochemical properties, especially associated with the presence of an electron-conductivity providing agent in the composite electrodes, to be obtained in a single step, which method does not adversely affect the density of these electrodes and the adhesion at the electrode/electrolyte interfaces within these composite electrodes.